It has been accepted practice for years to use one of four methods to calibrate near carrier spectrum analysis equipment. The first method requires the use of a known signal that can be substituted for the unknown signal. The signal used as a substitute is one that can be easily modulated directly. After the substituted signal provides the calibrated sideband band levels, the unknown signal is then reconnected for testing. A second method is a comparison technique where the discriminator is calibrated against a directly calibrated local oscillator-Intermediate Frequency discriminator of known response. The third method is to use the Bessel function nulling technique. The fourth method is to use circuitry (sometimes called an Armstrong circuit) to set up modulation sidebands that can be accurately added to the unknown signal to establish a calibration sideband pair.
The substitution technique is undesireable because it involves changing the hardware configuration between the calibration operation and the measurement operation thereby opening the way to all types of incidental errors that are very difficult to identify and correct. This causes the validity of the calibration to be open to incidental error and hence provides a questionable operation. The comparison method is unacceptable because it involves a considerable amount of manipulation that requires careful work and opportunity for many errors. Since the number of tasks required is rather large, the opportunity for error is greatly enhanced. It is also a very time consuming operation and cannot be done rapidly. The Bessel function technique is the best technique provided the source under test can be suitably modulated. The problem is that only a very few sources can be modulated in the required manner. Thus this technique is of limited usefulness. The fourth or so called Armstrong technique is easy to use and directly applicable to most cases as a means of calibration. The basic idea with this approach is to use a circuit as shown in FIG. 1, inserted between the source A and near Carrier Analyzer B to create the sideband pair. FIG. 2 depicts a typical signal with some type of modulation that represents the source to be measured. A sample of the source signal is coupled by directional coupler C through an attenuator D for amplitude control and applied to a mixer F as the local oscillator drive. A separate audio oscillator G that has variable amplitude and frequency controls is also applied to this mixer to form a sideband pair with reduced carrier amplitude as depicted in FIG. 3. The signal structure of FIG. 3 is coupled through phase shifter E, for phase control, to directional coupler where it is summed with the carrier from isolator J in the directional coupler H to produce the carrier-to-sideband relationship shown in FIG. 4. The phase shifter can create an in-phase or Amplitude Modulation, a quadrature or Frequency Modulation, or a combination of the two modulation forms. It is important to note that all the schemes discussed above generate double sideband modulation.